Gas operated firearm with gas piston surrounding a tubular magazine



Oct. 20, 1959 R. L. HILLBERG 2,909,101

GAS OPERATED FIREARM WITH GAS PISTON SURROUNDING A TUBULAR MAGAZINE Filed March 22. 1954 5 Sheets-Sheet 1 imm Oct. 2 0, 1959 R. l.. HILLBERG 2,909,101

GAS OPERATED FIREARM WITH GAS PISTON suRRouNDING A TUBULAR MAGAZINE Filed March 22, 1954 5 SheersfsheeI 2 .IE Ym ,7m/enfer jeoberl' ,[.Hz'llberg @iid Oct. 20, 1959 R. L. HILLBERG 2,909,101

GAS OPERATED FKIREARM WITH GAS. PISTON suRRouNDING A TUBULAR MAGAZINE Filed March 22. 1954 5 sheets-sheet s nu wlw .7m/enfer Zober' L .Hz'lberg Oct. 20, 1959 R. L. HILLBERG 2,909,101

GAs OPERATED FIREARM WITH GAS PISTON SURROUNDING A TUBULAR MAGAZINE Filed March 22. 1954 5 Sheets-Sheet 4 m/endr foben" L Hillbag Mq-M ' dgys.

Oct. 20,1959 R. L. HILLBERG GAs oPERATGn FIRGARM wrm GAs PIsToN suRRouNnING A TUBULAR MAGAZINE Filed March 22. i954 5 Sheets-Sheet 5 TIME SEC.

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United` States Patent Oi-ic@ 2,909,101 Patented Oct. 20,V 1959V if GAs'oPERA'IEDFmEARM WITH GAs'PISToN SURROUNDING A TUBULAR This invention pertains to self-loading firearms. More especially it. pertains to small arms of the gas operated type adapted to ,re ammunition ofA different power or charge but of the same gauge or caliber, wherein the various reloading operations 4are accomplished automatically following discharge of the firearm. A

IGas operated semiautomatic small arms developed heretofore, particularly in the case of sporting guns such as Shotguns, have been subjectto many disadvantages from both theoperational `as well as production standpoints. .This has been all the more acute where a gun is to be usedwith ammunition of diiferent powers for the same bore. One of the chief problems in designing a gun of this type, wherein a neat Vappearance `as well as good balance and Vproper feel in the hands of the shooter are to be achieved, is the placement of the gas piston Iandcylinder assembly used for actuating the gun. Asis typical in gas voperated self-loading guns generally, -a `portion of the, gas developed on discharge of a round is bled 01T. from the gun barrel to a cylinder, where its energy is: eonverted by reaction upon a piston to rnechanical movement' for effecting thesteps of unlocking thebreech blockV or bolt after firing, withdrawing the latter from battery position, ejecting the spent shell, compressing areturn spring for returning the bolt lto battery and advancing a fresh shell into position to be chambered in the breech on the forward motion of, the bolt. The difficulty arises from the practical necessity, or at least thev great desirability in` many instances, of placing on the undersideof the gun barrela tubular maga- "zine, forr'holding the desired number of rounds of am- -munition Commonly, and particularly inthe case of shotguns, there is no other really suitable or satisfactory manner of placing this component without destroying the compactness of the gun or its balance `and sighting characteristics. Other types of magazines, such as box magazines, have various disadvantages which render them unsuitable.

;Facedwith these prerequisites, gas operated semiautoi matic small arms developed heretofore have met the problem of where to place the gas cylinder assembly in various waysf One has been to place it at the forward end ofthe tubularV magazine, in which position it is substantially axially aligned therewith and consequently permits, `to some extent, retention ofcompactness of design. Such 'a gun, however, has a tendency to be nose healvy. A further and more serious Vditliculty with the foregoing `arrangement is that gas bled olf from the barrel to the operating cylinder at that point has already undergone substantial reduction in pressure, since the cylinder and l piston are disposed at a point relatively remote from the breech chamber. Therefore, in order to obtain a suficient force to operate -the gun rapidly and completely throughV ajcycle of operation, the cylinder and piston ,must'be of a relatively large size to provvide the needed piston area, and thev gas port in the barrel leading to the operating cylinder must also be relatively large to cessive admission into the operating cylinder of carbon, particles of lead, dirt or the like, which will interfere with the operation of the gun.

Another solution, where the operating cylinder is placed' at a point on the barrel closer to the breech chamber to take advantage of the higher gas` pressure available there has been to place the piston and cylinder at one side 0r the other of the gun, alongside the tubular magazine. This resul-ts in an awkwardV arrangement since the forearm of the gun must bemade quitebulky and thick at substantially the very point at which it is convenient for the shooter normally to grasp and support ,the gun with his forward hand when in shooting stance.A This againis boundvto reflect in the accuracy, ease and rapidity with which the gun can be aimed and red. p, n

lt has also been proposed among other expedients in the prior art, more specially in heavy ordnance guns, to employ a cylinder and piston mounted around the barrel of the gun itself. While this may help toward obtaining the advantages of better gas pressures, it also leads to difliculty both inV appearance and balance, and in the presence of an undesirable projection above the uppper surface of the barrel, which complicates the matter of providing a satisfactory sighting arrangement on a sporting arm. .Y

The present invention is directed. among other things to overcoming the foregoing disadvantagesin physical arrangement of theoperating cylinder, while. permitting it to be positioned at an optimum point on the gun barrel withrespect to the gas pressures obtaining therein. In general, the invention in this respect comprises the arrangement of an annular cylinder Aand piston assembly which encircles the tubular magazine of the gun and cooperates with it.to provide an operating unit for effecting the cyclingV of the various componentsnecessary to reload the gun. The novel annular cylinder and piston arrangement retains the advantages of a tubular magazine, overall compactness and balance of the gun in general and. nonin-terference of the operating cylinder assembly with the upper or sighting surface of the gun, while allowing for positioning of the cylinder at any selected. point along the extent, of the barrel. =For this reason primarily, the novel gun. is readily capable of rng all normal `types of ammunition from the highest power down to Vthe lowest power commonly available commercially with assured positiveness and rapidity of action.

Trouble has also beenrexperienced in prior types of gas operated guns because of jamming, sticking or similar improper functioning of the gas piston due to deposits of burnt powder, other carbonaceous material or particles of lead shot on the piston or its cylinder, with the result that the assured operation of the gun for any extended period of time without frequent disassembly and cleaning has been impractical.

VThe gun here disclosed also incorporates a simple but effective design of the operating cylinder Vand piston whereby any carbonized powder particles or particles of the projectile or shot, which are carried into the actuating cylinder and deposited on thewalls thereof and on the piston, are effectively removed on each actuation of the mechanismso that the working surfaces of thecylinder and piston are kept clean and sticking is avoided. v

Other features and advantages ofthe gunincluderuggedness of design, fewer operating parts and simplification thereof with resultant greater ease of assembly and disassembly. This in turn leads to increased capability of production and lower cost of the firearm. Other adments comprising the invention. Since the problems here concerned arise more especially in Shotguns, where ammunition of Varying powers for any given gauge are 3 available and are commonly employed, the specific description given hereinafter is of that type of gun. The principles, however, are equally applicable to ries.

In the drawings,

Fig. 1 is a partial view inf side elevation of a shotgun, parts being broken away to show certain of the operating components more clearly;

Fig. 2 is a view similar to Fig. 1 but from the opposite side of the gun; Y

Fig. 3 is a detailed side elevational View on enlarged scale and partly in section of the gun receiver and breech bolt mechanism, the bolt being shown in closed or battery position;

Fig. 4 is a similar view, the bolt being shown in its fully retired position;

Fig. 5 is a'partial View in elevation of the cartridge lifter arrangement;

Fig. 6 is a top plan view of the trigger assembly including the lifter;

Fig. 7 is a side View of the trigger assembly showing engagement of the bolt slide with the hammer for cocking the latter during retraction of the bolt;

Fig. 8 is a fragmentary plan view of the magazine and gas cylinder assembly, taken on line 8-8 of Fig. 2;

Fig. 9 is an enlarged fragmentary cross-sectional view of the gas cylinder and piston arrangement, taken in the vertical plane passing through the axes of the gun barrel and tubular magazine;

Fig. 10 is a View similar to Fig. 9, the piston being shown in its retired position in which it is completely withdrawn from the cylinder;

Fig. 11 is a cross-sectional view of the gun taken on line 11-11 of Fig. 9;

Fig. 12 is a perspective view of 4the annular piston;

Fig. 13 is a perspective view of the cylinder member forming the other wall of the annular cylinder arrangee ment;

Fig. 14 is a diagrammatic representation of a typical pressure-time relationship in the gun barrel following discharge of low and high power types of ammunition of the same caliber or bore; and

Fig. 15 is a graphical representation of the ratio of average total pressure to pressure difference against time, as derived from the graph of Fig. 9.

Before going into detail on a specific gun construction, it will be well to consider briefly some of the factors which must be borne in mind in designing a gas operated, self-loading gun of the type under discussion. As an aid to understanding some of the problems, there lis presented in Figs. 14 and 15 of the drawings a diagrammatic representation of the pressure-time relationships in a typical gun barrel immediately following discharge of both low power and high power types of ammunition of the same gauge. While the abscissa of these graphs are plotted in units of time, it will be appreciated that there is a direct relationship between time and the positions of the projectile charge or wad along the barrel after firing. Owing to inertia of the projectile charge, movement of the latter along the barrel does not begin at the very instant of firing of course. Thus, points on the graph of Fig. 14 slightly to the left of the peak pressures indicated will correspond roughly with the breech chamber, while points to the right thereof correspond with points successively nearer the muzzle.

As will be seen from the graph in Fig. 14, there is a very substantial difference in peak pressure produced on discharge of the two types of ammunition. Tn order to reduce to the maximum extent possible the size of the gas port leading from the interior of the gun barrel to the operating cylinder, and hence reducing the likelihood of introducing'particles of burned powder, lead or other dirt into the` operating cylinder, it is desirable torplace the latter at or near a point of maximum pressure lon the gun barrel. However, since placement of the gas take-olf port and cylinder at or very close to the breech chamber would result, in automatic guns of this type, in unlocking the breech mechanism prematurely, the gas port must be spaced from the breech chali'iber sufciently to allow the explosive charge to build up pressure enough, and for a time long enough, to accelerate. the projectile charge to the required speed before' the' bolt is withdrawn from breech-closingk position. Where a gun is designed to fire only one type of ammunition,.the gas port would accordingly be positioned at a point on the barrel as close to such maximum developed pressure as is practical, bearing in mind the above-mentioned necessary spacing for a momentary delay in opening the breech. In the graph of Fig. 14, this position is represented at a, at which a pressure difference of d1 exists betweenthe two types of ammunition, and the average total pressure is p1. Assuming-that the gun design is based on the pressure developed by the high power ammunition the use of lower power ammunition in that gun will not effect a full retirement of the bolt in the receiver and the gun will accordingly not function properly. Onthe other hand, if the design is based on the peak pressure developed by low power ammunition, it will be apparent from the foregoing that the use of high power ammunition in that gun will cause an excessively hard retirement of the bolt which can quickly lead to breakage or other damage to components of the gun. And if the average total pressure is used in this case, there being such a large pressure difference at point a, the reaction will be too small in one instance and too great in the other for satisfactory operation.

Conversely, it will be seen that from the standpoint of minimum pressure difference alone, a point on the gun barrel close to the muzzle, such as that represented by point b in Fig. 14 where the pressure difference is d2, would be desirable. But `it will also be noted that the average total pressure at this point, e.g. p2, is very low, in fact so low that it is then necessary to increase both the size of the gas port and of the piston area in the operating cylinder to an unreasonable extent in order to obtain sufficient force to effect operation of the gun mechanism. This of course leads to the previously mentioned diiculties of cumbersomeness and liability to excess accumulation of deposits in the operating cylinder, and the attendant jamming of the gun.

Where a gun is to be used for firing varioustypes of ammunition ranging rather widely in explosive or propellent power, therefore, it is necessary to select a point on the barrel for positioning the operating cylinder such that the ratio of the'average total pressure to the difference in pressure at the corresponding point is as high as is practicable.

Referring to the graph in Fig. 15, in which the foregoing ratio has been computed for various points on the graph of Fig. 14 and this ratio plotted against time,k it-will be apparent that, for the conditions there represented, the opti-mum positions will lie between the points x and y on the abscissa, where a secondary maximum or peak inthe ratio appears. Points to the left of x are impractical for the reasons discussed hereinabove, and points to the right of y are considerably less desirable owing to theY relatively low average total pressure andthe diiculties attendant upon use of such lower pressures. As will be seen from Fig. 15, the optimum condition occurs at about 0.0016 second, and reference back to Fig. 14 will show Ythat this corresponds to point c where the average total pressure p3 is relatively high, while the difference in pressures is still comparatively small.' Y Y It vwill generally then be found, however, that this desired point on the' barrel occurs intermediatev the extent ofthe subadjacent tubular magazine which, for the various vreasons pointed out hereinabove, it is desired to employ. Because of the interference caused bya-the magazine, placement ofthe gas cylinder assembly at this point has heretofore not been considered practical.Y By virtue of the novel arrangement of gas operating cylinder disclosed herein however, the Yadvantages of ,both the tubular type of magazine and the desired positioning of the gas cylinder can be obtained aswill now be explained.

Referring now particularly to Figs. l and 2 of the drawings; there is illustrated a self-loading shotgun 20 comprising a'receiver 21, a shoulder stock 22, a barrel 23 (parts of each of these last two members being shown as broken away for simpliflcationof thedrawings), a tubular magazine-24, aforearm 25, a' breech block assembly 26 and a trigger assembly27. A retaining bolt (not shown), passing longitudinally through the shoulder stock into a threaded socket 23 (see Figs. 3 and 4) in the rear face 29 of the receiver, secures the stock to the receiver in conventional manner. The barrel 23 and magazine 24 are also secured to the receiver in conventional manner, threaded openings 30, 31, respectively, being provided for this purpose inthe forward face 32 of the receiver. Also, the receiver has the usual longitudinal opening or action chamber 33 within which the breech bolt and trigger assembly are mounted. Shoulders 34 formed on either side of the action chamber at the forward portion thereof, and the upper surfaces 35 at the sides of trigger assembly 27, form tracks on which the slide 36 and bolt 37 are carried for reciprocation between the battery positionV of the breech block assembly 26, shown in Fig. 3, and its retired position adjacent the rear of the receiver, as shown in Fig. 4. A tiring pin 37a is 'carriedin bolt 37 in conventional manner to be struck by a hammer described presently and hre a cartridge in the chamber. The breech block assembly 26, in the example here shown,iis of the compound action type in which the bolt 37 has a heel 3S projecting upwardly at its rear end for locking the assembly in battery position by engagement in a notch 39 in the roof 40 of the receiver.' This is accomplished during the nal forward movement of the slide `36after the face 41 of the `bolt 37 has come into abutment with the rear fac'e 42 of the ring chamber. As can best be seen inFigs. 3 and 4, bolt 37 is carried by the slide 36, the bolt having a transversely extending camming lug y43 which seats in a mating transverse.slot y44 in -the slide when the assembly is out of battery. A toggle link `45 servesto connect the rear of the bolt to the slide, pivot pins 46 passing through the bolt vand 'slide -to secure the upper and lower ends of the link, respectively, to these members. During rearward travel Yof the 'slide 36, link 45 pulls the bolt rearwardly and down until the camming lug I43 nests in slot `44 so that the rearwardly directed, substantially vertical face 47 of the slot abuts a similarly disposed forward face Y48 of the lug. .Onthereturnimovement of the slide, the bolt is pulled -forward but remains nested in the slide until the aforesaid abutment of its forward face against the rear of the tiring chamber occurs. Being unable to go farther with further movement of the slide, the rear of the bolt is cammed upwardly by the camming lug 43 on ythe rear sloping face 49 of itscooperating slot 44, this being permitted by the notch 39 in Ithe roof of the receiverinto which the heel 38 of the bolt can then rise. The slide continues forward until the toggle link 45 passes over dead center and the lug 43 'rests onthe rear shoulder of the Vslot 44, whereby the bolt is locked in tiring position.

The trigger assembly 27, shown more especially in Figs. v6 and 7 and constituting the iire control mechanism of the gun, is of generallyconventional construction and will not be Vdiscussed vin any great detail here for that reason. Theassembly is secured within the action chamber of the receiver by, means of push pins 50 which pass through aligned holesin oppositecheeks or side walls of the receiver and the` respectively cooperating holes ofthe trigger plate 4assembly/within which sleeves or bushings 51 are provided. The trigger assembly also includes, of

course, a hammer 52, hammer yoke 53, ahammer spring 54 recessedrin an aper-ture 55 in the-rear of the plate,

"axsear, 56 and-a` trigger 57. The assembly alsozcarries with it aycartridge, lifter 58 which is pivotally supported @on `the sleeve 51 surrounding the forward push pin 50, whereby" the lifter projects forwardly fromvthe` trigger V"6 plate. In its normal position, it Vserv`es"as a floor for theaction'chamber of the receiver. See Figs. 1` to 4. As shown more especially in Fig. 5, a cartridge S delivered upon the lifter 58 from the tubular magazine, in .wellknown manner upon retraction of the breech block assembly .26, is raised by the lifter on the return movement of the bolt assembly toa position in which the nose ofthe bolt engages the rear cap of cartridge S and slides the latter forward into the breech chamber. This lifting is accomplished in timed relation to`the action of the bolt by means of a cammed pawl 59 carried on a'pivot 60 in the triggerV assembly. The pawl`is urged by engagement under its shoulder `61 of a spring 62 so as to eiect clockwise rotation of theY pawl as seen in Fig. 5, thereby tending to maintain contact`betweenitscam lobe 63 and a projecting nose V64 of the lifter.'A Thelifter is likewise biased by a spring V65 to pivot its cartridge-carrying end downwardly. Upon retraction of the bolt and slide assembly 26, the pawl is pivoted counterclockwise (Fig. 5) so lthat the rear of slide 36 Ypasses over the upper end of the pawl. A notch 66 in the under surface of the slide allows the pawl to spring back partiallyafter the rear of the slide has passed over Vitl Then, on the returny movement of the slide, the pawl is, positively rotated in a clockwise direction by engagement of the rear shoulder of notch 66'against the head of the pawl. This 'causes the pawl lobe 63 to depress the nose 64 of the lifter, Vthereby elevating the opposite end of lthe lifter and thecartridge carried by it. After the rear of the slide passes forwardly over the depressed pawl, the latter springs back again to its original position. Spring 62 is employed in the construction here illustrated as a means for retaining the sleeve 51 in the trigger plate assembly and as a det'ent means for retaining the push pin 50 in the sleeve,in addition to its function in biasing the pawl 59. This is ac.- (complished, as best seen in Pigs. 3 and 4, bypassing the spring through a slot 67 in the cheek of the receiver adjacent the end'of the sleeve, and there bending it to cause it to encircle the sleeve and pin. The spring is recessed at this end in a counterbore 68 concentric with the pin. Sleeve 51 has a transverse slot cu-t through its wall and extending partially around its periphery Within which 'the terminal portion `69. oftheV spring lies.'V This portion of the spring thus intersects the surface ofthe pinV 50 when -the latter is pushed into Vthe sleeve and is sprung outwardly thereby. By providing an annular groove adv jacent that end of the pin into which portion V69 of the spring may slip, resilient retention of the pinis accomplished.- Engagement of the spring in the slot of sleeve 5l serves to retain the latter in position.

The mechanism for accomplishing the cycling of the breech block assembly 26 in vorder to reload the gun automatically following discharge of a shell includes, as the principal component, a gas cylinder and piston assembly 70. 'Ihis latter is annular in construction, encircling the tubular magazine 24 and being positioned along its length at the point where the optimum gas pressure exists in the barrel, as previously discussed. A helical cornpression spring 71 surrounding the magazine and 'conlined between the rear of the piston 72 and the forward face 32 of receiver 21, and an action bar 73 disposed'at the left-hand side of the gun in the valley'formedV between the barrel and the magazine, complete the operating components of this mechanism. The forwardV end of the action bar is attached to the piston 72 while the rear end of the bar is attached to, slide 36 of the breech bolt mechanism.

The cylinder assembly 70 is made up in part of an outer member 74 secured to the underside of barrel 23 at .the selected point. The barrel is provided at this point with one or more gas ports on holes-75, three being shown in the-illustration in Fig. ll, which'cornmunicate with a cooperating or registering port or passage 76 provided in the Vhead-77 by which the cylinder memberV 74 is `secured to the barrel.- Y Port 76 provides communication between the interior of the cylinder member 74 and the gas ports 7Sr ofthe barrel. lCylinder member 74 has a main body portion: 78 which encircles the tubular magazine and is radially spaced therefrom.. The cylinder member also has a radially inwardly extending. wall 79 and a collar or sleeve 80 coaxial with the main body portion, which collar snugly engages the periphery of the tubular magazine 24. Thus the` main body portion 78 ofthe cylinder member provides the outer wall of an annular cylinder, while the inner wall of such cylinder is formed by the surface of the tubular magazine 24.

Cooperating with the cylinder` thus formed is the sleevelike piston 72 which is' supported on the surface of the magazine and capable of reciprocation axially thereon. The piston has a head portion comprising peripheral, laterally projectingrlands 81 of relatively short extent axially -of the piston. The inner and outer surfaces of these lands constitute the working surfaces of the piston within the cylinder, these surfaces being spaced away from the inner and` outer surfaces of the central body 82 of the piston which are thus kept out of. contact with the walls of the cylinder. At the rear of the piston, a skirt 83 is provided with an inwardly directed flange 84 which bears on the surface of the tubular magazine and serves both as a guide for that end of the piston and as a means of maintaining the spacing of the body 82 from the surface-of the magazine and the walls vof the cylinder. Spring 71, mentionedhereinabove, is in compression between the forward. face 32 of receiver 426 and the flange 84 of the piston, thereby urging the piston forwardly into the cylinder, as shown in Fig. 9. The piston is, however, prevented from bottoming, that is, abutting against the radial wall 79 of the cylinder member 74, by the action bar 73, as will be explained presently, so that there remains a small chamber at the head of the piston 72 into which gas expands upon discharge of shell. Entry of the gas into this chamber causes the piston to be retired against the force of the compression spring 71, completely withdrawing it from the cylinder member 74 -fto the position shown in Fig.- 10. iIn so doing, the piston forces the action bar 73 rearwardly on the magazine. Coupling of the action bar to the piston is made by a transversely extending curved finger 85 at the forward end of the bar which ts'into a slot 86 (Fig. 12) in the skirt 83 of the piston. As will be seen from Figs. 9 and l0 more particularly, finger 85 is normally confined in slot 86 against vertical displacement by the under surface of the barrel 23 and the upper surface of the tubular magazine 24. `By reason of the curvature of the finger so that it conforms to the curved periphery of the magazine, it thus acts as a hook preventing displacementof the forward end of the bar laterally of the gun. Attachment of the rear of the action bar tothe breech bolt assembly 26 is accomplished by means of a notch 87 in the bar (see Fig. 2) which mates with a lug or projection 88 of slide 36, whereby the reciprocating action of the the piston 72 is transmitted through action bar 73 to the breech bolt assembly 26 to slide the latter to and from battery position within the receiver `21. Vertical and lateral displacement of the rear of action bar 73 is prevented by its passage into a rabbeted slot 89 in the receiver (see Figs. 2 and 8).

Upon firing a round in the gun, gases entering the cylinder cause the piston to start its retirement and the momentum thus built up thencarries the breech block assembly to its rearmost position in the receiver, compressing the return spring 71. When the momentum is expended, spring 71 acts to return the piston toits forward position again. Such action of course serves also to pull the breech block assembly 26 forward into battery position through the interconnection of the action bar 73. When the bolt 37 reaches its fully forward position, slide 36 is prevented from further forward travel and this preventaction bar 73 Vfrom moving farther forward. also. The'length vof the action bar is accordingly adjusted so `that'when this lattercondition isI reached, piston 72 is 8 prevented from bottoming in the cylinder so that the aforesaid spacing of the head of the piston from the end or radial wall of the cylinder is provided. f

As has been Amentioned previously, the accumulation of burned powder, or other carbonaceous material, or of particles of lead, on the head of the piston in typical gas operated firearms of prior construction has occasioned much troubledue to sticking or jamming'of the piston in the cylinder, unless frequent occasion is taken to disassemble the ygun and remove such deposits. In the preferred embodiment of the invention illustrated in the accompanying drawings, provision is made to remove such deposits automatically on each cycle of operation. This is accomplished by undercutting the annular cylinder walls within which the piston 72 operates so that adjacent the forward, closed end of the cylinder, sharp-edged shouldersV are disposed past which the lands 81 of the piston move on each cycle of operation. This produces a shearing action longitudinally of the piston rwhich serves to scrape off any of the undesired deposits which may be accumulating on the head of the piston and also possibly along the working surfaces thereof due to blow-by.

Referring more particularly to Figs. 9 and l0, it will be seen that the tubular magazine 24 has an annular groove or cut 90 in its periphery at the forward end of the cylinder which provides a sharp-edged shoulder 91 at the inner working surface of the cylinder. Similarly, the body 7 8 of the outer cylinder member 74 is undercut to provide a groove or channel 92 and afsharp-edged shoulderV 93. These shoulders shear off carbonaceous deposits tending to form on the working surfaces of lands 81 of the piston. The inner andouter shoulders 94, 95 respectively, of the lands 81 (both fore and aft) are likewise substantially square-edged and act to keep the surface of 'the magazine and the inner wall of cylinder member 74 clean. In order also that deposits which have been scraped free from the working surfaces will not accumulate between the central body portion 82 and the tubular magazine 24, that portion of the piston has a number of holes 96 which permit such deposits to fall free of the piston and magazine. it will also be noted that the piston 72 is completely Withdrawn, in its cycle of operation, from the cylinder assembly so that any deposits which have been scraped off into the cylinder itself will be blown free by the entering gases as soon as the head of the piston leaves the cylinder on the following cycle. These deposits fall into the lower part of the forearm 25 where they are completely out of the way'of any of the working parts. lIri order also that the accumulation in the forearm does not become excessive, the lat-ter is slotted at one or more points to provide an aperture 97 through which these deposits may then fall entirely free of the gun (see Figs. l and 2).

As shown in the accompanying drawings, the outer cylinder member 74 is provided with a head 77 which engages the under surface of the barrel 23 where it is brazed in position. Thus the member serves not only as a part of the annular cylinder for the piston 72, but also as a support for the forward or outer end of Ithe tubular magazine 24. As has been mentioned, the magazine is secured in conventional manner to the forward face 32 of receiver `21 by screwing the magazine into the socket 31 in the body of the receiver. This being so, the tit between collar of the cylinder member 74 and the outer periphery of the tubular magazine 24 must accordingly be `such that the magazine is capable of being slid longitudinally or 'axially therethrough and then turned to engage its threads with the threaded aperture of the receiver. `It will also be apparent of course that the fit must be suiciently tight so that excessive blow-by of gases entering the cylinder does not occurbetween the collar 80 and the surface of the'tubular magazine. VThis can of course be reduced by making the ycollar 80 of greater length axially of the magazine. Also it may be accomplished by providing a slightoutward flare of the 9 tube atits Aouter end so that it tendsftb seat securely in the collar 80.

While it has not been specificallymentioned,` it will also be apparent that the forearm 25 of the gun is adapted to be assembled therewith by sliding it longitudinally of the gun barrel toward the receiver in conventional fashion until its inner end abuts against an escutcheon 1007at the forward face 32 of the receiver. Thereafter a retaining cap 98 is screwed onto the outer end of the tubular magazine 24 so that its skirt presses against the forward end of the forearm'and connes the latter securely on the gun. The cap 98 also serves'to hold the follower spring 99 under compression, all in conventional manner, so that the follower will advance cartridges S from the magazine into the receiver. Suitable cartridge stopmeans, not shown, disposed in the receiver and actuated in timed relation with the reciprocation of the breech block assembly, controls the delivery of the cartridges by the follower.

Various modiiications in the specific construction described herein may obviously be made within the concept of the invention. And as mentioned previously, the

l. In a gas operated firearm having a receiver, breechV bolt mechanism mounted for reciprocation to and from battery position in said receiver including means for locking said mechanism in battery, a barrel secured in said receiver and having a breech chamber therein, the combination which comprises a tubular magazine mounted subadjacent said barrel and parallel thereto, said magazine having coupling means at its inner end for releasable engagement with said receiver to permit axial withdrawal of said magazine, a cylinder member secured to said barrel and encircling said magazine in radially spaced relation to the latter, an end wall on said member extending radially inward into snug engagement with the periphery of said tubular magazine while permitting axial sliding movement thereof through said member, said member and magazine dening an annular cylinder concentric with said magazine and opening toward said receiver, an annular piston slidably carried on said magazine for vreciprocation into and out of said cylinder, and resilient means normally urgingsaid piston into said cylinder, an `action bar connecting said piston to said breech bolt mechanism for reciprocation of the latter thereby, said barrel having a gas take-olf port extending through its wall and a registering passage in said member leading to the interior of said cylinder adjacent said cylinder wall to allow propellent gases to enter said cylinder when said firearm is fired and to eect momentary retirement of said piston against said resilient means.

2. In a gas operated small arm having a receiver, breech bolt mechanism reciprocable to and from battery position in said receiver, and a barrel secured to a forward face of said receiver, the combination which comprises a tubular magazine mounted beneath and in alignment with said barrel, said magazine being threaded at its inner end and said receiver having a cooperating threaded aperture within which said magazine is axially received, a cylinder member secured to said barrel forwardly of said receiver through which Vmember said magazine passes, said member having a cylinder wall portion spaced radially from said magazine and an end wall which extends radially. inward from said portion into snug engagement with the periphery of said tubular magazine while permitting axial sliding movement of said magazine through said member, said magazine and member thus defining an annular cylinder concentric with said magazine and opentoward said receiver, an annular 10 piston slidably carried n'said magazine for rre'ciprocation into and out of said cylinder, and resilient means normally urging said piston into said cylinder, an actionbar connecting said piston to said breech bolt mechanism for reciprocation of the latter thereby, said barrel having a gas take-off port extending through its wall into communication with the interior of said cylinder adjacent said end wall. t I

3. A firearm as defined 'inVclaim-Z, whereinv the interior of said cylinder wall portion and the exterior of said tubular magazine are each peripherally undercut adjacent the closed outer end of said cylinder to provide sharpedged shoulders past which the inner and outer cylindrical surfaces of said piston move during reciprocation to shear olf carbon or similar deposits tending to form on said surfaces.

4. A iirearm as defined in claim 3, wherein the inner and outer surfaces of said annular piston are undercut to provide sharp-edged shoulders which wipe past the working surfaces of said cylinder during reciprocation to shear off carbon or similar deposits tending to form on said surfaces, and said piston is provided with an aperture in the side Wall thereof, wherby deposits scraped loose from the surface of said tubular magazine may be discharged through said aperture when it is retired to a point outside of said cylinder on reciprocation of said piston.

5. A firearm as defined in claim 2, wherein the inner and outer surfaces of said annular piston are undercut to provide sharp-edged shoulders which wipe past the working surfaces of said cylinder during reciprocation to shear olf carbon or similar deposits tending to form on said surfaces, and said piston is provided with an aperture in the side wall thereof, whereby deposits scraped loose from the surface of said tubular magazine may be discharged through said aperture when it is retired to a point outside of said cylinder on reciprocation of said piston.

V6. In a gas operated irearm having a-receiver, breech bolt mechanism mounted for reciprocation to and from battery position in said receiver, ire control means associated with said breech bolt mechanism for controlling the discharge of said firearm, a barrel secured to a forward face of said receiver and a tubular magazine secured to said receiver below and in substantial alignment with said barrel, the combination which comprises an annular cylinder member positioned along said magazine intermediate its ends, said cylinder member being secured to said barrel at its underside and having an inner surface-spaced radially from said magazine, a collar adjacent one end of said cylinder member which snugly engages the periphery of said tubular magazine while permitting axial sliding movement thereon, whereby said cylinder member and said tubular magazine cooperate to form an annular cylinder closed at one end; an annular piston mounted on said tubular magazine for sliding movement thereon into and out of said annular cylinder, said piston having a head portion adapted and arranged to make a close sliding fit with the internal annular surfaces of said cylinder; a gas port in said gun barrel and a cooperating one in said cylinder member which provide communication between -the interiors of said barrel and annular cylinder, said barrel port being disposed at a point intermediate the longitudinal extent of said tubular magazine; a helical spring on said magazine, said spring being coniined between said piston and the forward face of said receiver to urge said piston into said Y 11 engagement ,by the adjacent' surfaces of the bar-rel and 2,558,364 magazine. Y 2,596,841 2,685,754- References Cited 1n the le of this patent UNITED STATES PATENTS 5 785,971 Mcclean Mar. 2s, 1905 421,613 966,995 Brown Aug. 9, 1910 702,757 1,157,571 Moench Oct. 19, 1915 880,415 2,482,880 Sefried Sept. 27, 1949 123,901

12 Koucky June 26, 1951 Davidson May 13, 1952 Critendon Y Y Aug. 10, 1954 FOREIGN PATENTS Germany Nov. 16, 1925 Germany Feb. 15, 1941 France Dec. 28, 1942 Sweden Jan. 25, 1949 

